(1) Field of the Invention
The invention concerns an apparatus for coupling laser light into a combustion chamber of a combustion engine including a combustion chamber window and a structural element, wherein the combustion chamber window is releasably fixable to the structural element. The invention further concerns a laser spark plug, a cylinder of a combustion engine and a combustion engine including an apparatus of the aforementioned kind.
(2) Description of Related Art
Laser ignition is at the present time in an intensive development phase, wherein at the present time besides the actual laser technology involved a great deal of attention is being placed on the engine aspect, in particular capability of being implemented in mass production. The principle of laser ignition is based on an intensive laser pulse being introduced into the combustion chamber of the engine and there focused on a focal point. At that focal point the intensity exceeds a threshold value which is sufficient to ignite a plasma spark. That plasma spark, in a similar fashion to the spark of conventional spark ignition, is capable of igniting a fuel-air mixture in the combustion chamber of an engine.
For use in relation to combustion engines, the laser ignition concepts which at the present time are being most intensively pursued are such that the laser pulse is generated by a solid state laser which is integrated together with the optical coupling-in and coupling-out system in a housing fixed to the cylinder head. That unit, by analogy with conventional spark ignition, is referred to as the laser spark plug. The ignition laser is generally optically pumped by a semiconductor laser connected to the laser spark plug by way of an optical fiber. The pumping operation, during which excitation of the laser-active atoms in the solid state crystal is effected until build-up and discharge of the laser pulse occurs, lasts for about 200 μs-400 μs. The ignition pulse itself lasts for a few nanoseconds.
The optical coupling-in system for coupling the laser pulse into the combustion chamber of the combustion engine comprises a suitable lens system and what is referred to as the combustion chamber window representing the last optical element before the beam passes into the combustion chamber.
The advantage of laser ignition over conventional spark ignition is inter alia that the ignition spark can be placed freely into the depth of the combustion chamber where optimum ignition conditions exist. In contrast thereto combustion initiation with conventional spark ignition takes place in the immediate proximity of the combustion chamber wall, with the flat electrodes which define the ignition spark impeding formation of the flame core. The energy of the laser spark can be greatly increased by increasing the power output of the laser system without thereby involving increased wear, as occurs for example with spark ignition due to electrode wear.
A further advantage of laser ignition is that, with increasing engine power output, the required minimum pulse energy (which is the energy of the plasma spark which is required as a minimum for ignition of the fuel-air mixture) decreases. In comparison the conventional spark ignition systems noticeably reach the system limits, at the engine power output levels which are planned in the future.
The main problems in regard to designing and mass-production implementation of laser ignition include inter alia ensuring or maintaining the optical properties of the combustion chamber window over the service life of the engine. Especially in relation to the combustion chamber-side interface of the combustion chamber window, high thermo-chemical loadings and the deposit of solid residues from the combustion process can lead to clouding of the surface, whereby both the beam is attenuated (that is to say partly absorbed) and also it is scattered, which leads either to a considerable reduction in the energy of the plasma spark or however also failure of the plasma spark.
The above-described problem is usually combated by on the one hand providing reserves for losses and attenuation phenomena due to the service life, by means of high levels of pulse energy, and on the other hand endeavoring to achieve the effect of burning the window surface free by virtue of the high levels of pulse power. The disadvantage of those procedures lies in considerably increased costs for the high laser power output required for that purpose and the high specific loading on the optical interfaces in particular of the combustion chamber window. DE 10 2005 043 963 A1 and U.S. Pat. No. 4,422,323 describe apparatuses of the kind set forth in the opening part of this specification. In those apparatuses the combustion chamber window can be replaced.